Geotechnical aspects of domestic on-site effluent systems (septic tanks) and their impact on groundwaters are discussed and the limited relevant literature is reviewed. While there are few Australian case studies, the early stages of an ongoing study at Dodges Ferry, Tasmania, show a connection between shallow aquifer quality, number of residences and domestic on-site wastewater disposal practices. Of 26 groundwater samples analysed, a number fail to meet established criteria for potable use. Most samples were highly or very highly saline. This renders them unpalatable and has an adverse impact on vegetation if the water is used for irrigation. Several samples had pH less than 6.0 and those taken from shallow wells were discoloured by leached soil organic matter. Elevated nitrate levels, an indicator of contamination by sewage effluents, were found at nine locations and these were usually associated with small clusters of residences. One very high sample was clearly contaminated by effluent from an adjacent on-site wastewater disposal system. One odorous, black organic seep was found at the foot of the dunes backing a bathing beach and this gives cause for concern that failing on-site wastewater disposal systems are contributory to reduced bathing water quality. Faecal coliforms and Escherichia coli were not found, even in samples with the highest nitrate concentrations, suggesting that residence time has been sufficient for bacterial die-off. Contamination of shallow aquifers is greatest where there is a high density of residences with small lots.